1. Field of the Invention
The present invention relates to an image forming method and an image forming apparatus, and more particularly, to an image forming method that forms images by thermally transferring, for example, ink of a thermal transfer ink ribbon onto a medium to be recorded and then by recording the images by transferring the images onto the medium using heat and pressure, and to an image forming apparatus.
2. Description of the Related Art
Conventionally, in representing a multi-gradation using a color material, such as a thermal transfer ink ribbon or an ordinary thermal paper, on which only two gray-scale values per unit pixel can be represented in a thermal printer for forming images by controlling the intensity of a current passing through a thermal head, the images with the multi-gradation are obtained through a halftoning process, such as a dither method or an error diffusion method. However, the dither method has a disadvantage in that a dither matrix becomes large and the resolution becomes lowered as the number of gray-scale values to be desired increases. In addition, the error diffusion method has been used for a line thermal printer (see Japanese Unexamined Patent Application Publication No. 2002-86786). However, the error diffusion method is not common for a thermal printer has a great effect of accumulated heat on a thermal head.
In addition, there has been proposed a method of representing a multi-gradation by controlling an area per dot recorded on a recording medium, using a print system including an ink ribbon having an adjusted application thickness of ink, a recording medium having a substrate and a porous-surface layer formed on the substrate, the porous-surface layer having a specific range in size, a thermal head provided with a plurality of heating resistors formed in a line at regular intervals, temperature gradient of each of the heated heating resistors being such that temperature is the highest at a middle portion thereof and becomes lower toward ends thereof, and a gray-scale control circuit for controlling ink fusion areas heated by the heating resistors by controlling the intensity of a current passing through the thermal head (see Japanese Patent No. 2850930 and Japanese Unexamined Patent Application Publication No. 2000-85171) This method enables multi-gradation images with higher resolution and higher quality as compared to the dither method. However, the method has a restricted application to only a special porous recording medium.
On the other hand, a method of forming images on a variety of recording media by first forming the images on an intermediate transfer medium having an image receiving layer and an adhesive layer and then by transferring the images on a recording medium using heat and pressure has already been put to a practical use (see Japanese Unexamined Patent Application Publication No. 2002-79765) According to this method, there is no effect on the intermediate transfer medium even if the recording medium is changed. Also, since this method may consider only transferability of the images onto the recording medium, it allows formation of the images on various kinds of recording media as compared to a direct printing method. In addition, in this method, since a heat roller made of soft heat-resistant rubber is used to transfer the image onto the recording medium, it is possible to form the images on a recording medium having more or less unevenness.
In addition, there has been proposed a melt type color thermal transfer recording method in which a thermal head having a plurality of heating elements arranged therein in a main scanning direction is used, a recording paper moves relative to the thermal head at least in a sub-scanning direction vertical to the main scanning direction, driving of each of the heating elements is controlled during the relative movement, the length of ink dots in the-sub scanning direction changes depending on the density of pixels to be recorded, and a halftone color image is recorded on the recording paper by cyan, magenta, and yellow ink dots, wherein at least cyan and magenta are so arranged as to be different in their densities of pixels in the sub-scanning direction (see Japanese Patent No. 2577115 and Japanese Unexamined Patent Application Publication No. 5-116356). This method can prevent the variation of the color tone of images and the occurrence of pattern moirs due to any deviation of color registration in the sub scanning direction.
However, the porous-surface layer is required to be formed on a surface of the recording medium in order to form the images by the dot area gradation. On the other hand, in the method of forming the images using the intermediate transfer medium, the porous-surface layer is not formed since an intermediate transfer layer to be transferred must be transparent and smooth. Accordingly, in the method of forming the images using the intermediate transfer medium, the images must be formed using a sublimation thermal transfer method that can represent gradation for each dot or the dither method or the error diffusion method using the thermal transfer ink ribbon, or must be formed on the smooth intermediate transfer layer by unstable and variable narrow dot area gradation. In addition, in this method, when the dot gradation is recorded on the intermediate transfer medium having no porous-surface layer, dots are formed with weak printing energy when forming images at low density. Accordingly, it is difficult to form the dots reliably, unstable spots may occur in the images, and images having granularity may occur. Moreover, since the images are apt to be affected by deviation of characteristics of the thermal head, irregularity of sensitivity of the intermediate transfer medium, environments, such as ambient temperature, it is difficult to achieve high-quality images.
The sublimation thermal transfer method can obtain a smooth gradation characteristic and allows images to be formed with high precision since this method does not require an area gradation by a dot modulation. However, since dye-based ink is used in this method, a problem may occur in the durability of the dye due to mismatch with the recording medium, thus requiring to choose a proper recording medium.
In addition, in the method in which at least cyan and magenta are arranged so as to be different in their densities of pixels in the sub-scanning direction, since the density of images is represented by varying the length of dots, the number of gray-scale values is too small to record images of photographic quality, and accordingly, it is difficult to achieve high-quality images.
Accordingly, it is an object of the present invention to form images on a medium to be recorded with sufficient gradation representation and high resolution using thermal transfer ink.